"Dogs are very intelligent and adaptable creatures who, like countless others, lack the cognitive development necessary to self-recognize visually, whether in a mirror, on a video, or in a photo," Liz Stelow, an animal behavior clinician at the Veterinary Medical Teaching Hospital of the University of California, Davis, said by email.

Studies that have tested whether dogs could use mirrors as tools to find food or their owners had mixed results.

It's not too surprising: Dogs evolved to communicate through scent, and smell is "more important for dogs than a visual recognition of 'self,'" Stelow noted.

"It takes a good deal of sophisticated integration of information about yourself and your own movements and what you're seeing in front of you in that glass," Reiss said.

That's why it takes human babies 18 to 24 months to catch on to what they're seeing in the mirror.

Amazingly, some animals have also cracked the code: Dolphins, elephants, magpies, and some great apes know they're looking at themselves in the mirror.

Animals that show mirror self-recognition will often go through phases of discovery. One phase is what Reiss calls "the Groucho stage," in which animals repeat odd movements as they "seem to figure out that their behavior and the behavior of that guy in the mirror is related."